And then I would add to..

The length of the flash hole from the powder is not near as critical IF the breech plug has the ability to 'BREATH' for the lack of a better word.

The plug on the right will ignite BH just as reliable as the one on the left. In addition the plug on the right will go longer before the cleaning of the 'flash channel' is necessary because it can contain more volume.



I am not an artist or a mechanical wizard but here is the basic drawing that I tend use.





This is the modification of a Triumph breech plug. The Triumph plug is really long and a factory plug will ignite BH without a problem unitl the 1/8" flash channel becomes clogged.

I modified mine by opening the flash channel with a #21 bit and installing the vent liner in the nose of the plug. Ignition was great even with the coolest primers.